Glass-ceramic composition for recording disk substrate

ABSTRACT

A glass ceramics composition for recording disk substrate consists essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO 2 , from 3 to 15 wt % of Al 2 O 3 , from 3 to 15 wt % of Li 2 O, from 0.2 to 5 wt % of P 2 O 5 , and from 0.1 to 0.8 wt % of ZrO 2 .

RELATED APPLICATION

[0001] This application is based on application No. 11-191743 filed in Japan, the content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a glass ceramic composition, more particularly, relates to the glass ceramic composition suitable for magnetic disk substrate.

DESCRIPTION OF THE PRIOR ART

[0003] Magnetic disks are mainly used as recording media of computers. Aluminum alloys have heretofore been used as the material of magnetic disk substrates. However, in the recent trend for a smaller size, a thinner thickness, and a higher recording density of magnetic disks, a higher surface flatness and a higher surface smoothness are increasingly desired. Aluminum alloys cannot satisfy the desire, and a material for magnetic disk substrates which can replace aluminum alloys is required. Thus, in particular, recent attention has been focused on the glass substrate for the disk because of its surface flatness and smoothness and excellent mechanical strength.

[0004] As glass substrates for disks for recording media, there have been proposed a chemically reinforced glass substrate having a surface reinforced by ion exchange or like method and a glass ceramics substrate on which a crystal component has been precipitated to reinforce the bonding. In recent years, the latter crystallized glass substrate in which a crystallite has been precipitated in glass by heat treatment has drawn particular attention because of its excellent strength and high productivity.

[0005] As recent requirements on the performance of a disk for a recording medium have been more stringent, a substrate material has also been required to have an increased strength related directly to the bending or warping of the disk during high-speed rotation. The strength can be represented by the elastic modulus ratio (=Young's modulus/specific gravity) of the substrate material. The elastic modulus ratio having a higher value indicates a higher mechanical strength. However, a glass-ceramics composition conventionally known has the problem that the productivity thereof is reduced significantly if the strength thereof is to be increased.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a glass ceramic composition which is suitable for use in an improved glass substrate for a recording medium.

[0007] Another object of the present invention is to provide a glass ceramic composition which has high productivity irrespective of its high elastic modulus ratio.

[0008] Still another object of the present invention is to provide a disk substrate for a recording medium which has high productivity irrespective of its high elastic modulus ratio.

[0009] Thus, the present invention provides a glass ceramics composition for recording disk substrate consisting essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3 to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010] The present invention provides a glass ceramics composition for recording disk substrate consisting essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3 to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.

[0011] In the composition, SiO₂ is a glass network former oxide. The melting properties deteriorate if the proportion thereof is lower than 65 wt %. If the proportion thereof exceeds 80 wt %, the composition becomes stable as glass so that the crystal is less likely to be precipitated.

[0012] Al₂O₃ is a glass intermediate oxide and a component of an aluminum borate crystal, which is a crystalline phase precipitated by heat treatment. If the proportion of Al₂O₃ is lower than 3 wt %, the crystal is precipitated in reduced quantity and a sufficient strength is not achieved. If the composition rate of Al₂O₃ exceeds 15 wt %, the melting temperature is increased and devitrification is more likely to occur.

[0013] Li₂O is a fluxing agent and a component a component of a lithium disilicate crystal, which is a crystalline phase precipitated by heat treatment. If the proportion of Li₂O is lower than 3 wt %, the precipitation amount of lithium disilicate crystal is insufficient. If the composition rate of Li₂O exceeds 15 wt %, a lithium disilicate crystal, which is a crystalline phase precipitated by heat treatment is not stable so that the crystallization process cannot be controlled. In addition, the chemical durability is reduced, which may affect a magnetic film, while the stability in the polishing to cleaning steps is lowered.

[0014] P₂O₅ is a fluxing agent and a nuclear forming agent for precipitating a silicate crystal, which is an important component for uniformly precipitating the crystal over the entire glass. If the proportion of P₂O₅ is lower than 0.2 wt %, satisfactory nuclei are less likely to be formed so that crystal grains are increased in size or the crystal is precipitated non-uniformly. Consequently, an extremely small and uniform crystal structure is less likely to be obtained and a flat, smooth surface required of the glass substrate as a disk substrate cannot be obtained by polishing. If the proportion of P₂O₅ exceeds 5 wt %, the reactivity of the glass in a molten state to a filter medium is increased and the devitrifiability thereof is also increased, so that productivity during melt molding is reduced. In addition, the chemical durability is reduced, which may affect a magnetic film, while the stability in the polishing to cleaning steps is lowered.

[0015] ZrO₂ is a glass intermediate oxide and a nuclear forming agent. In particular, ZrO₂ holds the precipitation of a quartz crystal in crystal. If the proportion of ZrO₂ is lower than 0.5 wt %, satisfactory crystal nuclei are less likely to be formed so that crystal grains are increased in size and the crystal is precipitated non-uniformly. This prevents the obtention of an extremely small and uniform crystal structure and the obtention of a flat, smooth surface by polishing, which is required of the glass substrate as a disk substrate. In addition, the chemical durability and the migration resistance are reduced. This may affect a magnetic film and degrades stability in the polishing to cleaning steps. If the proportion of ZrO₂ exceeds 0.8 wt %, the melting temperature is increased and devitrification is more likely to occur during melt molding, which lowers productivity. Moreover, the precipitated crystalline phase changes so that desired characteristics are less likely to be obtained.

[0016] Besides the above-mentioned basic components, CaO as a fluxing agent can been added. By adding CaO serving as a fluxing agent, a melting property and a stable crystal phase are improved. If the proportion of CaO is lower than 0.1 wt %, a melting property does not sufficiently improve. If the proportion thereof exceeds 5 wt %, the composition becomes stable as glass so that the crystal is less likely to be precipitated and a sufficient strength is not achieved.

[0017] Besides the above-mentioned basic components, K₂O as a fluxing agent can been added. By adding K₂O serving as a fluxing agent K₂O, the production stability is improved. If the proportion of K₂O is lower than 0.1 wt %, however, the melting properties are not improved sufficiently. If the proportion of K₂O exceeds 5 wt %, the glass becomes stable and the crystallization is suppressed, while the chemical durability is reduced. This may affect a magnetic film and degrades stability in the polishing to cleaning steps.

[0018] Besides the above-mentioned basic components, Sb₂O₃ as a fluxing agent can been added. By adding Sb₂O₃ serving as a fluxing agent, production stability has been improved. If the proportion of Sb₂O₃ is lower than 0.1 wt %, however, a sufficient clarifying effect can not be achieved and the productivity is lowered. If the proportion of Sb₂O₃ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled so that required characteristics are less likely to be obtained.

[0019] Besides the above-mentioned basic components, B₂O₃ as a former can been added. By adding B₂O₃ serving as a former, the phase splitting of the glass is promoted and the precipitation and growth of the crystal are promoted. If the proportion of B₂O₃ is lower than 0.1 wt %, the melting properties are not improved sufficiently. If the proportion of B₂O₃ exceeds 15 wt %, devitrification is more likely to occur and molding becomes difficult, while the crystal is increased in size, so that an extremely small crystal is no more obtained.

[0020] Besides the above-mentioned basic components, MgO as a fluxing agent can been added. By adding MgO serving as a fluxing agent, the crystal in the form of grains aggregates to form an aggregation of crystal grains. If the proportion of MgO is lower than 0.1 wt %, the range of operating temperatures is narrowed down and the chemical durability of a glass matrix phase is not improved. If the proportion of MgO exceeds 12 w %, another crystalline phase is precipitated so that it becomes difficult to achieve a desired strength.

[0021] Besides the above-mentioned basic components, BaO as a fluxing agent can been added. By adding BaO serving as a fluxing agent, production stability has been improved. If the proportion of BaO is lower than 0.1 wt %, however, the melting properties are not improved sufficiently. If the proportion of BaO exceeds 5 wt %, the glass becomes stable and the crystallization is suppressed so that it becomes difficult to achieve a desired strength.

[0022] Besides the above-mentioned basic components, ZnO as a fluxing agent can been added. By adding ZnO serving as a fluxing agent, it helps uniform precipitation of the crystal. If the proportion of ZnO is lower than 0.1 wt %, however, the uniformity of the crystal is not sufficiently improved. If the proportion of ZnO exceeds 5 wt %, the glass becomes stable and the crystallization is suppressed, so that required strength is less likely to be achieved.

[0023] Besides the above-mentioned basic components, Nb₂O₅ as a fluxing agent can been added. By adding Nb₂O₅ serving as a fluxing agent, a material serving as a crystal nucleating agent is increased. If the proportion of Nb₂O₅ is lower than 0.1 wt %, the rigidity is not sufficiently improved. If the proportion of Nb₂O₅ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that desired characteristics are less likely to be obtained.

[0024] Besides the above-mentioned basic components, Ta₂O₅ as a fluxing agent can been added. By adding Ta₂O₅ serving as a fluxing agent, the melting properties and strength are improved, while the chemical durability of the glass matrix phase is improved. If the proportion of Ta₂O₅ is lower than 0.1 wt %, however, the rigidity is not sufficiently improved. If the proportion of Ta₂O₅ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that desired characteristics are less likely to be obtained.

[0025] Besides the above-mentioned basic components, La₂O₃ as a fluxing agent can been added. By adding La₂O₃ serving as a fluxing agent, the precipitation of the crystal is suppressed. If the proportion of La₂O₃ is lower than 0.1 wt %, however, the rigidity is not improved sufficiently. If the proportion of La₂O₃ exceeds 5 wt %, the crystallization of the glass becomes unstable and the precipitated crystalline phase cannot be controlled, so that required properties are less likely to be obtained.

[0026] Next, a description will be given to a fabrication method. Raw materials containing the main components of the glass substrate to be finally produced are sufficiently mixed in specified proportions. The resulting mixture is placed in a platinum crucible and caused to melt. The molten product is cast in a metal mold so that it is formed into a rough configuration and annealed to a room temperature. The molten product is then held at a specified temperature for a specified time during a primary treatment (heat treatment) such that crystal nuclei are formed. Subsequently, the molded mixture is held at a specified temperature for a specified time during a secondary heat treatment such that crystal nuclei grow. By slowly cooling the molded mixture, an objective crystallized glass is obtained.

NUMERICAL EXAMPLES

[0027] A description will be given next to specific numerical examples incorporating the embodiments. In Table 1: the proportions (unit: wt %) of materials composing the glasses of the examples 1-4; the melting temperatures and times; the primary heat treatment temperatures and times; the secondary heat treatment temperatures and times; the main precipitated crystalline phases; the subordinate precipitated crystalline phases; the mean diameters of the crystal grains; the specific gravity s: the Young's moduli; and the specific moduli. Likewise, the glasses of the examples 5-8 are shown in Table 2. Likewise, the glasses of the examples 9-12 are shown in Table 3. Likewise, the glasses of the examples 13-16 are shown in Table 4. Likewise, the glasses of the examples 17-20 are shown in Table 5. Likewise, the glasses of the examples 21-23 are shown in Table 6. TABLE 1 Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 SiO₂ 75.2 74.0 76.0 75.0 Al₂O₃ 6.5 10.4 10.5 10.5 Li₂O 8.5 11.0 9.0 9.5 P₂O₅ 4.0 4.5 3.8 4.0 ZrO₂ 0.5 0.1 0.7 0.8 CaO 2.5 0.2 K₂O 1.8 Sb₂O₃ 1.0 Melting Temperatures (° C.) 1460 1440 1440 1440 Melting Times (hours) 2.50 2.50 2.50 2.50 Primary Heat Treatment 570 570 575 570 Temperatures (° C.) Primary Heat Treatment Times 5.50 5.50 5.50 5.50 (hours) Secondary Heat Treatment 690 700 690 700 Temperatures (° C.) Secondary Heat Treatment Times 2.50 3.00 2.50 3.00 (hours) Main Precipitated Crystalline Lithium Lithium Lithium Lithium Phase disiliate disiliate disiliate disiliate Subordinate Precipitated Quartz Quartz Quartz Quartz Crystalline Phase Mean Diameters of the Crystal 0.06 0.06 0.06 0.06 Grains (μm) Specific Gravity 2.40 2.21 2.28 2.28 Young's Module 93.0 88.0 88.1 88.1 Specific Module 38.7 39.8 38.7 38.7

[0028] TABLE 2 Exam- Exam- Exam- Exam- ple 5 ple 6 ple 7 ple 8 SiO₂ 74.0 73.0 69.0 75.0 Al₂O₃ 10.5 11.0 12.0 10.0 Li₂O 9.0 10.6 12.0 9.5 P₂O₅ 3.3 4.2 2.3 3.9 ZrO₂ 0.5 0.7 0.2 0.6 CaO 2.7 K₂O 0.5 4.5 Sb₂O₃ 1.0 Melting Temperatures (° C.) 1440 1440 1440 1460 Melting Times (hours) 2.50 2.50 2.50 2.50 Primary Heat Treatment 575 570 580 575 Temperatures (° C.) Primary Heat Treatment Times 5.50 5.50 5.00 5.50 (hours) Secondary Heat Treatment 700 700 720 700 Temperatures (° C.) Secondary Heat Treatment Times 3.00 3.00 4.00 3.00 (hours) Main Precipitated Crystalline Lithium Lithium Lithium Lithium Phase disiliate disiliate disiliate disiliate Subordinate Precipitated Quartz Quartz Quartz Quartz Crystalline Phase Mean Diameters of the Crystal 0.08 0.08 0.08 0.08 Grains (μm) Specific Gravity 2.29 2.25 2.23 2.34 Young's Module 88.0 88.0 87.0 87.2 Specific Module 38.4 39.0 38.9 37.2

[0029] TABLE 3 Exam- Exam- Exam- Exam- ple 9 ple 10 ple 11 ple 12 SiO₂ 69.0 74.0 71.0 74.0 Al₂O₃ 10.5 9.5 11.4 11.0 Li₂O 11.0 10.0 9.0 9.6 P₂O₅ 5.0 3.8 4.2 4.2 ZrO₂ 0.5 0.7 0.4 0.7 Sb₂O₃ 4.0 B₂O₃ 2.0 4.0 MgO 0.5 Melting Temperatures (° C.) 1460 1440 1440 1440 Melting Times (hours) 2.50 2.50 2.50 2.50 Primary Heat Treatment 570 575 570 570 Temperatures (° C.) Primary Heat Treatment Times 5.50 5.50 5.50 5.50 (hours) Secondaiy Heat Treatment 720 700 700 700 Temperatures (° C.) Secondary Heat Treatment Times 4.00 3.00 3.00 3.00 (hours) Main Precipitated Crystalline Lithium Lithium Lithium Lithium Phase disiliate disiliate disiliate disiliate Subordinate Precipitated Quartz Quartz Quartz Quartz Crystalline Phase Mean Diameters of the Crystal 0.08 0.08 0.08 0.08 Grains (μm) Specific Gravity 2.56 2.23 2.20 2.27 Young's Module 88.0 87.0 86.2 86.8 Specific Module 34.4 39.0 39.1 38.3

[0030] TABLE 4 Exam- Exam- Exam- Exam- ple 13 ple 14 ple 15 ple 16 SiO₂ 72.0 76.0 72.0 75.0 Al₂O₃ 10.0 9.5 10.0 9.5 Li₂O 9.5 9.0 9.5 9.2 P₂O₅ 3.7 3.7 4.3 3.6 ZrO₂ 0.3 0.8 0.2 0.7 MgO 4.5 BaO 1.0 4.0 ZnO 2.0 Melting Temperatures (° C.) 1440 1460 1500 1460 Melting Times (hours) 2.50 2.50 2.50 2.50 Primary Heat Treatment 575 575 570 575 Temperatures(° C.) Primary Heat Treatment Times 5.50 5.50 5.50 5.50 (hours) Secondary Heat Treatment 700 690 700 700 Temperatures (° C.) Secondary Heat Treatment Times 3.00 2.50 3.00 3.00 (hours) Main Precipitated Crystalline Lithium Lithium Lithium Lithium Phase disiliate disiliate disiliate disiliate Subordinate Precipitated Quartz Quartz Quartz Quartz Crystalline Phase Mean Diameters of the Crystal 0.08 0.08 0.08 0.08 Grains (μm) Specific Gravity 2.23 2.38 2.62 2.34 Young's Module 87.0 87.2 91.0 88.0 Specific Module 39.0 36.7 34.8 37.6

[0031] TABLE 5 Exam- Exam- Exam- Exam- ple 17 ple 18 ple 19 ple 20 SiO₂ 76.0 77.0 76.0 75.0 Al₂O₃ 7.6 9.7 8.6 13.3 Li₂O 9.8 9.6 9.5 9.0 P₂O₅ 3.3 2.5 2.0 1.5 ZrO₂ 0.3 0.7 0.4 0.7 ZnO 3.0 Nb₂O₅ 0.5 3.5 Ta₂O₅ 0.5 Melting Temperatures (° C.) 1460 1440 1460 1460 Melting Times (hours) 2.50 2.50 2.50 2.50 Primary Heat Treatment 575 580 580 585 Temperatures(° C.) Primary Heat Treatment Times 5.50 5.00 5.00 5.00 (hours) Secondary Heat Treatment 690 690 690 700 Temperatures (° C.) Secondary Heat Treatment Times 2.50 2.50 2.50 3.00 (hours) Main Precipitated Crystalline Lithium Lithium Lithium Lithium Phase disiliate disiliate disiliate disiliate Subordinate Precipitated Quartz Quartz Quartz Quartz Crystalline Phase Mean Diameters of the Crystal 0.08 0.08 0.08 0.08 Grains (μm) Specific Gravity 2.34 2.29 2.44 2.34 Young's Module 88.2 87.8 90.0 88.8 Specific Module 37.7 38.3 36.8 38.0

[0032] TABLE 6 Exam- Exam- Exam- ple 21 ple 22 ple 23 SiO₂ 74.0 74.0 77.0 Al₂O₃ 12.5 11.0 10.5 Li₂O 8.0 10.6 7.3 P₂O₅ 1.5 3.5 2.5 ZrO₂ 0.2 0.7 0.2 Ta₂O₅ 3.8 La₂O₃ 0.2 2.5 Melting Temperatures (° C.) 1520 1440 1460 Melting Times (hours) 1.75 2.50 2.50 Primary Heat Treatment Temperatures (° C.) 585 575 580 Primary Heat Treatment Times (hours) 5.00 5.50 5.00 Secondary Heat Treatment Temperatures 700 700 690 (° C.) Secondary Heat Treatment Times (hours) 3.00 3.00 2.50 Main Precipitated Crystalline Phase Lithium Lithium Quartz disiliate disiliate Subordinate Precipitated Crystalline Phase Quartz Quartz Lithium disiliate Mean Diameters of the Crystal Grains (μm) 0.08 0.08 0.08 Specific Gravity 2.76 2.27 2.52 Young's Module 91.0 86.2 89.0 Specific Module 33.0 38.0 35.4

[0033] Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modification depart from the scope of the present invention, they should be construed as being included therein. 

What is claimed is:
 1. A glass ceramics composition for recording disk substrate consisting essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3 to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.
 2. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of CaO.
 3. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of K₂O.
 4. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Sb₂O₃.
 5. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 15 wt % of B₂O₃.
 6. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 12 wt % of MgO.
 7. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of BaO.
 8. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of ZnO.
 9. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Nb₂O₅.
 10. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Ta₂O₅.
 11. A glass ceramics composition as claimed in claim 1, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of La₂O₃.
 12. A recording disk substrate made of glass ceramic, wherein the glass ceramics composition for recording disk substrate consisting essentially, expressed in terms of weight percent on the oxide basis, of from 65 to 80 wt % of SiO₂, from 3 to 15 wt % of Al₂O₃, from 3 to 15 wt % of Li₂O, from 0.2 to 5 wt % of P₂O₅, and from 0.1 to 0.8 wt % of ZrO₂.
 13. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of CaO.
 14. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of K₂O.
 15. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Sb₂O₃.
 16. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 15 wt % of B₂O₃.
 17. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 12 wt % of MgO.
 18. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of BaO.
 19. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of ZnO.
 20. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Nb₂O₅.
 21. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of Ta₂O₅.
 22. A recording disk substrate as claimed in claim 12, wherein said glass ceramic composition consisting essentially, expressed in terms of weight percent on the oxide basis, of from 0.1 to 5 wt % of La₂O₃.
 23. A recording disk substrate as claimed in claim 12, wherein the glass ceramic includes crystalline phase of quartz.
 24. A recording disk substrate as claimed in claim 12, wherein the glass ceramic includes crystalline phase of lithium disilicate. 